Long hailed as the future of electronic ciphers, quantum cryptography has arrived. Swiss company id Quantique has introduced a commercial quantum cryptography system, American company MagiQ Technologies is developing a second, and at least one of the field's prominent researchers believes the U.S. government is already using the technology in classified applications.

Drawing on Werner Heisenberg's uncertainty principlethe notion that when you try to measure a quantum system, a system that involves very small particles, you alter it in such a way that your measurement is inaccuratequantum cryptography allows two people to exchange encryption keys over a public network. They can then use those keys to encode their correspondences, knowing that the messages are completely secure.

The two correspondents exchange keys by sending each other individual photons of light, using the polarization of each photon to indicate a binary digit. With help from a clever algorithm, the correspondents are able to glean enough information from the stream of photons to build a key.

Thanks to the uncertainty principle, anyone eavesdropping on a stream reads only gibberish and leaves telltale signs that he was there. In theory, if you encode an e-mail message, telephone call, or financial transaction using quantum techniques, its contents will be forever hidden from prying eyes.

Chris Fuchs, a quantum information researcher at Bell Labs, says government researchers in the field of quantum cryptography have fallen suspiciously silent recently. Fuchs believes that the government is already using the technology to trade classified messages, even though the technology currently works only over distances of a few dozen kilometers. "That's how far the White House is from the Pentagon," he says.

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